An airplane has flown across the United States, powered only by sunlight. The impressive feat was made possible by improvements in solar cell and battery technology. This chart, produced by the Department of Energy, shows how solar technology has improved:

(Lawrence Kazmerski, National Renewable Energy Laboratory)

The efficiency is the fraction of captured solar energy that is converted into electrical power. The Web site for the plane, named Solar Impulse, reports that the vessel was powered by monocrystalline silicon solar cells, represented by blue squares on the chart. Interestingly, this is not the most energy-efficient category of solar cells. However, Solar Impulse's designers were less concerned with absolute efficiency than they were with the ratio of efficiency to weight. The lighter the solar cells, the farther the plane could fly on each kWH of energy.

Because the flight across the United States took more than 24 hours (specifically, about 2 months), the plane needed to collect energy during the day and then use it at night. It used lightweight lithium polymer batteries to do this. The Solar Impulse team says the "total efficiency of the propulsion train"—counting power lost by the solar cells, the batteries and other components—was about 12 percent.

Solar-powered airplanes won't replace traditional airplanes any time soon. Airplanes powered by traditional jet fuel fly much faster and can carry much heavier loads. But the flight across the United States was an impressive feat of engineering. And the Solar Impulse team now has its sights on an even bigger goal: flying around the world on solar power.